Although static magnetic computer memories have been proposed since at least the 1960's, none have satisfactorily answered the need to provide a large amount of fast, inexpensive and reliable memory.
Today, the predominant use of magnetic memory is in connection with magnetic disks, tapes, drums and the like. For example, the disk drives in personal computers store large quantities of information on magnetic disks. However, all of these devices require an electromagnetic read/write head to be moved relative to the magnetic medium. Thus, although these magnetic media can store large amounts of information at relatively low cost, they are bulky, require maintenance, are very slow compared to other technologies such as random-access-memory (RAM) and consume substantial power to operate the motors.
Other magnetic memories include static magnetic memories such as core memory, magnetoresistive memory, inductive memory, bubble memory and Hall effect memory. Static magnetic memories do not use the relative movement of a magnetic medium and electomagnetic head. Instead, static magnetic memories use electrical current to change the information stored in the discrete magnetic elements. However, each of the aforementioned devices has its disadvantages. When information is read from most core memories, the data stored in the memory is destroyed. Accordingly, the information must be restored or refreshed every time it is read, which slows down the speed of the memory. In addition, core memory has a relatively large physical size and cost compared to the amount of information it can store, and a large amount of power is required to operate the memory. Inductive memory and bubble memory are also bulky and have large power consumptions. Bubble memory further suffers from being quite slow. Although magnetoresistive memory is very fast, it is rather difficult to measure the small change in resistance associated with stored data. In addition, most previous attempts at magnetoresistive memory yielded devices capable of storing only small amounts of data.
Other attempts have been made to overcome the deficiencies of the prior art. Dimyan, et al, U.S. Pat. No. 4,360,899, discloses a magnetic domain random-access-memory (RAM) which has magnetic storage elements disposed at the intersections of X and Y conductors. The X and Y conductors are perpendicular to one another but are essentially in the same plane. The magnetic element for each cell has a uniaxial magnetic anisotropy perpendicular to the plane of the read and write conductors. In operation, the value of a particular cell is set by passing current through the X and Y conductors which intersect at the cell. Depending on the direction of the current, the polarity of the magnetic element will either be up or down and perpendicular to the substrate. Detection is made in any manner readily known to those of ordinary skill in the art. Dimyan does not describe the exact magnetic material used in the cell element. Instead, the magnetic element is generally stated as having "uniaxial magnetic anisotropy perpendicular" to the substrate. Dimyan does refer to two patent applications as possible methods of manufacture, and one of those applications matured into U.S. Pat. No. 4,290,843. The '843 patent discloses growing magnetic garnet on selectively damaged portions of a substrate. The anisotropy presumably results from the crystal structure of the garnet.
Matthews, U.S. Pat. No. 5,075,247, discloses a method of making a Hall effect semiconductor memory cell. A grid of conductor lines overlie a substrate and magnetic patches are disposed in the squares defined by the grid. The planes formed by the conductors are usually above the magnetic patches. The polarity of the magnetic patches are perpendicular to the surface of the substrate. To write to the magnetic patch, current is passed in opposite directions through the four conductors surrounding the particular patch. This in turn sets the polarity of the patch to the desired orientation. The polarity of the patch is read by complicated non-destructive means integral with the device. Although various types of magnetic material are suggested, the reference does not appear to contemplate single domain magnetic materials because the magnetic patches can store domains other than up or down for analog data.
Kump et al., IBM Technical Disclosure Vol. 13, No. 7 at 2110 (December 1970) discloses a coupled NDRO magnetic film memory. The memory has word and bit-sense lines, a soft magnetic film layer exhibiting uniaxial anisotropy in the plane of the layer, a hard magnetic film layer also exhibiting uniaxial anisotropy, and a layer of insulation between the hard and soft layer. In operation, information is written to the film by applying current to the word and bit-sense lines which orients the magnetization in the films in one of two opposite directions. The memory is read out non-destructively by sending a pulse which is sufficient to rotate the magnetization in the soft layer, but not the hard layer.
Terman et al., IBM Technical Disclosure Vol. 8, No. 11 at 1598 (April 1966) discloses a non-destructive readout memory cell. The apparatus includes two magnetic films, one designated the read film and the other designated the storage film. When the storage film is magnetized in the 0 direction, the magnetization vector of the read film is bent in the same direction as the magnetic field used to read the film. Thus, during a read, no sense signal is received when the storage field is set to 0. When the storage field is magnetized in the 1 direction, the magnetization vector of the read film is bent at a right angle to the magnetic field used to read the film, thus inducing a sense signal. After the read pulse ends, the bias furnished by the storage film restores the read film magnetization angularity to its 1 position.
Tsuya et al., "Aluminate disc using oxidation", IEEE Transactions on Magnetic, Vol. MAG-22, No. 5 (September 1986) discloses filling a plurality of long and thin pores perpendicular to the planar surface of a disc with magnetic material for storing information. The magnetic properties of the disc are such that the hysteresis loop is tight and the magnetization vectors are perpendicular to the surface. The disk is apparently intended for use with standard magnetic disk reading apparatus.
Despite these prior attempts, there is still a need for a magnetic memory which is integrated (no moving parts), easily manufactured, fast, has a large storage capacity but small physical size and consumes only modest amounts of power. Although previous technologies may have incorporated each of these properties individually, none of the above-identified devices include all of these properties.